Susceptor

ABSTRACT

A susceptor for inclusion in an inductive heating system is built up from a plurality of modules consisting of rings of conductive material such as graphite. The modules are assembled so as to define a susceptor made of rings or similar structure spaced one from the other so that the susceptor structure has a substantially reduced mass.

United States Patent Metcalf et al.

[ SUSCEPTOR Inventors: James F. Metcalf, Willingboro; Robert H. Klingennan, Medford Lakes, both of NJ.

Assignee: Cragmet Corporation, Rancocas,

Filed: Oct. 5, 1970 Appl. No.: 77,789

U.S. Cl. ..2l9/l0.49, 13/27, 219/l0.51 .Int. Cl. ..H05b 5/00 Field of Search ..2l9/10.49, 10.51; 13/27 References Cited UNITED STATES PATENTS 12/1963 Morey ..219/l0.49

[451 Oct. 3, 1972 3,210,455 10/ 1965 Sedlatschek ..219/10.51 1,378,188 5/1921 Northrup 13/27 1,830,481 11/1931 Northrup ..13/27 2,889,438 6/1959 Macdaniels et a1.....2l9/10.49

Primary Examiner-R. F. Staubly Assistant Examiner-Hugh D. Jaeger Attorney-Seidel, Gonda & Goldhammer ABSTRACT A susceptor for inclusion in an inductive heating system is built up from a plurality of modules consisting of rings of conductive material such as graphite. The modules are assembled so as to define a susceptor made of rings or similar structure spaced one from the other so that the susceptor structure has a substantially reduced mass..

12 Claims, 5 Drawing Figures within the structure and heated by energy radiating from the structure. The heat radiating structure has come to be known as a susceptor..l n many instances a susceptor is required to heat objects made of a conductive material because direct application of heat through induced currents may cause warping or otherwise have deleterious efiects upon the object itself. There are, of course, many other induction heating processes where a susceptor is heated by induced currents to produce heat which is transmitted to the work load by radiation and in some instances by conduction.

The typical high temperature furnace with a susceptor consists of an induction coil surrounding the susceptor. The annulus between the susceptor and the induction coil is usually filled with lampblack or other thermal insulation to protect the coil. The susceptor itself is inductively heated by magnetically inducing currents therein. The interior of the susceptor defines a heat radiating surface which in turn forms a high temperature zone in which an object may be heated such as for sintering. The coil, susceptor and insulation are normally placed in a vacuum chamber to prevent oxidation of the susceptor itself. Operation in a vacuum may be desirable because an inert atmosphere, while sufficient to protect the susceptor from oxidation, introduces a cooling medium and therefore requires additional power to complete the heating process. However, inert gas is used for certain applications, such as where a low pressure and high temperature might cause sublimation of the carbon being heater. The sublimed carbon could affect the load object.

Typically, although not always, a susceptor for a high temperature furnace is made of graphite. The susceptor may in some instances be monolithic or may be made up of blocks or slabs joined either under pressure or by tongue and groove or the like. Similar construction techniques are used for assembling susceptors made of other materials. However, graphite, because of its extreme high temperature capability is normally the preferred substance for a susceptor. This is particularly true for high temperature graphitizing furnaces.

For heating small objects, the susceptor can be made by machining graphite from a solid bar. For larger sizes or rectangular shapes, blocks or plates are used. in any case, the resulting susceptor is a solid or continuous structure. Such a structure suffers from several disadvantages. Its large mass requires large amounts of energy to start up. Moreover, once brought up to temperature (e.g., 5,000F.) the stored potential energy represents quite a safety hazard should the protective insulation and other cooling devices fail.

Another disadvantage of massive susceptors is that the thermal potential energy must be removed before the chamber in which the furnace is operated can be opened. The amount of time required to cool exceptionally large susceptors can be as much as 18 days. Sometimes inert gases are introduced to accelerate the cooling process. However, a continuous susceptor insulated with powdered lampblack or its equivalent provides no ready circulation path for the cooling gases. Thus, the introduction of cooling gases is only moderately beneficial in accelerating the cool down process.

Inv addition to the thermal momentum of solid susceptors, another disadvantage is the inability to repair defects in the susceptor or its insulation without removal of the susceptor from the furnace and subsequent re-insulation after it has been repaired. Such complete disassembly of the furnace is costly in terms of equipment, down time, and manpower.

It therefore is an object of the present invention to provide a novel and unobvious susceptor which overcomes the foregoing disadvantages. Moreover, it is an object of the present invention to provide a susceptor of novel and unobvious construction which secures further unexpected results.

In accordance with the present invention the susceptor is constructed from individual modules built up and joined together so as to form a susceptor with substantially less mass.

In accordance with the present invention the susceptor is constructed from individual modules built up and joined together so as to form a susceptor with relatively less mass and with a large number of ventilating paths.

Still further in accordance with the present invention, a susceptor is made up of a number of modules which are joined together to form spaced rings of correct size for the frequency of the induction furnace. The rings are spaced apart in accordance with the heating pattern required.

More particularly, a susceptor in accordance with the present invention comprises a susceptor built up from individual pieces, such as bars of graphite, pinned or otherwise joined together to form rings of varying size and shape. These rings are spaced along a common axis, normally concentric with the induction coil. Appropriate bars interconnect the rings so as to provide a susceptor with a large number of ventilating openings.

A susceptor constructed in accordance with the principles of the present invention has several advantages. Since the susceptor is made of a number of distinct modular pieces, any portion of it can be removed and replaced with new modules. This greatly reduces the repair requirements. Still further, the susceptor need not be removed from the furnace in order to make a repair.

The mass of the susceptor is greatly reduced, thereby also reducing the cool down time by limiting the amount of potential heat energy that can be stored in the susceptor. In a like manner, the rate at which the susceptor can be cooled is greatly increased by providing additional ventilating openings. Stated otherwise, there is an increase in the surface area and coolant current paths.

Yet another advantage of the susceptor is that its resistance can be matched to source of magnetic energy (coil system), thereby optimizing magnetic coupling, efficiency, and power factor.

Yet another advantage of the reduced mass of the susceptor is that it requires less energy to bring it up to temperature. Thus it heats faster to operating temperature with a given power source. Moreover, there is a reduction in the amount of potential energy that must ultimately be dissipated on cool down.

A further advantage of a susceptor constructed in accordance with the present invention is that by selected positioning of the rings, power distribution within the susceptor can be controlled. Thus, the susceptor becomes a tool for providing selected temperatures along its length. By way of example, by providing closer spacing between rings the ends of the susceptor relative to the spacing at the middle of the susceptor, the effects ofv end losses are reduced, thus providing a uniform temperature gradient throughout the interior of the susceptor.

A heating furnace constructed using the inventive susceptor has several advantages. For example,

modules of carbonaceous felt can be used for insulation. This means that portions of the insulation can be replaced as compared with the existing requirement that all of the insulation be replaced at the same time. In particular, a susceptor constructed in accordance with a disclosed embodiment of the present invention provides access to the annulus between the coil and the susceptor for the replacement of portions of the insulation.

Still another advantage found in a furnace constructed in accordance with the present invention is that the insulators themselves can be constructed so as to aidin the cool down process.

Yet another advantage of a furnace using a susceptor in accordance with the present invention is that it can also be used as a resistance grid for resistance heating.

Yet another advantage of a furnace constructed in accordance with the present invention is that the insulation itself can be used as a radiating surface for providing heat energy to the object which is being temperature treated.

For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIG. 1 is a top plan view of a susceptor and a sectional view of a furnace in which the susceptor is installed taken along the line 1-1 in FIG. 2.

FIG. 2 is a vertical sectional view of the furnace and susceptor illustrated in FIG. 1 taken along the line 2- 2.

FIG. 3 is a large sectional view showing one means of joining the modular pieces which make up the susceptor.

FIG. 4 is an enlarged partial sectional view showing one means forjoining the rings of the susceptor.

F IG.-5 is an exploded view of the joining means illustrated in FIG. 4.

Referring now to the drawing in detail, wherein like numerals indicate like elements, there is shown in FIGS. 1 and 2 a furnace 10 having a susceptor l2 constructed in accordance with the principles of the present invention.

As shown, the furnace 10 includes a vacuum chamber 14 having an exhaust outlet 16 through which atmospheric gases may be pumped from the chamber 14. by appropriate apparatus (not shown).

Positioned within the chamber 14 is an induction coil 18 with appropriate support structure 20. As is conventional, the coil 18 is connected to a source of altemating current energy and generates a coaxial magnetic field. lfdesired, the electrical conductive coil 18 can be connected to a source of coolant such as water. Since the structure and operation of induction coils for heating is well known and described in the literature as well as in various patents, it need not be further described in detail.

The susceptor 12 is positioned within the induction coil 18 so that the magnetic field induces electric currents to flow within its structure. In the embodiment illustrated the susceptor 12 is concentric with the cylindrical, helically wound induction coil. Of course, it may be otherwise positioned depending upon the type of inductive coupling which is desired.

Insulation 22, to be described in more detail hereinafter, is positioned within the space between the susceptor l2 and coil 18. In the embodiment shown the space has a general shape of a cylindrical annulus. However, those skilled in the art will readily recognize that the space occupied by the insulation may take on other shapes, depending upon the shape of the coil and susceptor.

Referring now to FIGS. 2, 3, 4 and 5, the detailed structure of a susceptor constructed in accordance with the principles of the present invention is described. As best shown in FIG. 2, the susceptor is made up of a plurality of rings 24 vertically spaced from each other by varying amounts. Each of the rings 24 is in turn made up from a plurality of bars 26 which in the illustrated embodiment take the form of rods. The bars 26 are straight cylindrical pieces in the illustrated embodiment which are pinned together by tapered pins 28 as illustrated in FIG. 3. As thus constructed, each ring 24 is a polygonal structure. By way of example, the illustrated rings 24 are dodecagons. However, it should be understood that this is by way of illustration, not limitation since the rings can assume many other sizes or shapes for a closed loop or endless structure. For exam ple, they need not be regular polygons; they can be circular.

Each bar 26 in the illustrated embodiment has tapered holes formed in its ends and it is connected to an adjacent module by the tapered pin 28 which is frictionally fitted through aligned holes. The pin 28 is preferably made of the same material as the bars 26. The tapered pins provide good electrical contact. At the same time gravity enhances the contact. That is, expansion or contraction of the pins and bars is taken up by the gravitational force to maintain intimate contact. The bridging bars 26a always apply a downward force on pins 28 and bottom bars 26b. Also the tapered joints readily accommodate slight misalignments between bars 26a and 26b.

The rings 24 are spaced one from the other by interconnecting rods 30, each of which extends from ring to ring. The interconnecting rods 30 may be of varying length, depending upon the desired spacing between individual rings 24. 7

Each rod 30 is provided with a boss 32 machined into its end. Each boss 32 fits into a hole 34 extending through the bars 26b intermediate the ends thereof. A plane surface 36 is machined onto opposite sides of the hole 34. The surfaces 36 are machined so as to be parallel to each other.

The bosses of the interconnecting rods 30 are inserted into each of the holes 34 until the shoulder defined by the boss in the end of the rod mates with a plane, parallel surface 36 on opposite ends of the hole 34. This is best illustrated in FIG. 4. Thus, the interconnecting rods 30 provide electrical interconnection between the rings 24 as well as structural support and spacing for the ring. Basically, each rod 30 supports a bar 26b at its upper end. Intermediate rods are in turn supported by the next lower bottom bar 26b.

The purpose in using separate interconnecting rods 30 to support the bottom bars 26b in rings 24, rather than permitting the rods to serve the dual function of interconnecting the rings as well as the bars 26, is that such separate use makes the maintaining of parallelism between the various rings 24 much simpler.

In the embodiment described, the rings 24 are made up from individual bars pinned together as shown. However, it should be understood that the rings 24 could be otherwise constructed. For example, they could be solid circular rings, or semicircles pinned together somewhat in the manner described herein. In addition, other common joining techniques could be used in place of the pins 28, such as bridging pieces or tapered joints. Still further, the rings 24 could be separated by solid cylinders of lesser wall thickness. Such cylinders would provide a lower mass susceptor in accordance with the present invention. The cylinders could be integral with or separably connected to the rings 24. Solid cylinders may be used where the operating process of the furnace requires the use of lampblack as an insulator between the susceptor and the coil. Such a furnace would still be advantageous in that the rings could be designed to match the frequency and depth of penetration of the magnetic field while the solid cylinders support such rings and permit the use of powdered lampblack as an insulator. Such a susceptor would have a substantially reduced mass although it would not cool as fast as a susceptor in which the rings 24 are open at all sides.

Still further, it should be noted that the rings 24 illustrated in FIG. 2 are much closer together at the upper and lower ends of the susceptor 12 than they are in the middle. This is accomplished simply by providing shorter interconnecting rods 30. Thus, by manipulating spacing between discrete rings, it is possible to compensate for and reduce end losses by increasing the density of the susceptor at the ends compared to the middle. The result is a substantially uniform temperature gradient throughout the length of the susceptor. Of course, other temperature profiles are obtainable by proper spacing.

In addition to the rods 30, elongated rods 31 through the bridging bars 26a may also be provided. Rods 31 are preferably continuous rods extending through openings in the rods 26a from the bottom of the susceptor to the top of the susceptor. The purpose of the rods 31 is to provide independent vertical alignment with the bridging pieces 26a. Moreover, they provide lateral support.

As constructed, the susceptor is provided with a large number of openings or windows 38. The provision of these openings has several advantages. One advantage is that it permits the circulation of cooling gases all around each module making up the susceptor 12. By way of example, not limitation, inert cooling gases can be circulated through the chamber 14 by appropriate valving control on inlet 40 and outlet 42.

Another advantage of the openings or windows 38 is that it provides ready access to the insulation 22 positioned between the susceptor 12 and the coil 18. As previously indicated, insulation has heretofore taken the form of lampblack. By using a susceptor constructed in accordance with the present invention, other types of insulation can be used. For example, cylindrical rods of carbonaceous felt 44 are illustrated in FIGS. 1 and 2. Of course, the use of carbonaceous felt is by way of example, since other equivalent heat insulating materials can be used.

The openings 38 permit the heat absorbed by the insulation 22 to radiate back into the furnace and thus to serve as an aid in maintaining the temperature together with a susceptor. In addition, the cooling gases can now pass over the insulation 22 to help cool it much faster than was heretofore possible.

Still another advantage of constructing the insulation 22 from individual pieces 44 is that the pieces 44 can be individually removed and replaced as they become damaged or otherwise come into disrepair. This can be accomplished without removing the susceptor from within the coil. All that is necessary is that the insulation be pulled out through the openings 38 and then replaced through the same openings.

The bars 26, pins 28, interconnecting rods 30, and rods 31 may be conventionally made of graphite. However, it should be understood that the susceptor can be made up of other materials. The only requirement for materials which make up the rings 24 or other modular devices is that they be conductive and capable of withstanding the ultimate temperatures to be generated.

According to conventional principles of electrical engineering, the coil 18 induces current in the susceptor 12. Such an electrical structure inherently has a poor power factor. The susceptor of the present invention provides a means for optimizing the magnetic coupling, efficiency and power factor. Thus, since the structure is inherently adjustable in shape and size, it is possible to adjust such shape and size so as to optimize efficiency by matching the resistance of the susceptor to the reactance of the coil and its electrical supply. Such a match will also optimize the power factor.

In operation, the furnace 10 requires much less electrical energy to bring it up to temperature because of the reduced mass of the susceptor 12. As mentioned, cool down rates are greatly increased by reason of the lower mass of the susceptor 12. Moreover, because of the relatively low mass of the susceptor, the modules can be built up to make the susceptor practically any size that is desired.

Although the term module has been used to describe the rings made of rods 26, it should be understood that this is by way of example. The particular modules used to build up the susceptor structure can take other shapes, as desired.

From the foregoing, it should be obvious how the previously listed advantages of the present invention are accomplished.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

We claim:

1. A susceptor for radiant heating, said susceptor comprising interconnected conductive bars, certain of said conductive bars being connected to each other to define a ring, a plurality of such rings, and certain other of said conductive bars interconnecting said rings and spacing said rings one from the other.

2. A susceptor in accordance with claim 1 wherein said rings and spacing bars are assembled to define a generally cylindrical structure.

3. A susceptor in accordance with claim 1 wherein said rings are spaced closer to one another at certain portions of said structure than at other portions.

4. A susceptor in accordance with claim 1 wherein said rings are interconnected by said spacing bars to define a generally cylindrical structure, said rings adjacent the ends of said cylindrical structure being spaced closer together than the remaining intermediate rings.

5. A susceptor in accordance with claim 1 wherein said certain of said conductive bars connected to each other to define a ring are connected to each other by tapered pins extending into tapered holes in the ends of said conductive bars.

6. A susceptor in accordance with claim 1 wherein said spacing bars have means at their ends extending into openings in said certain of said conductive bars for supporting said certain of said conductive bars.

7. A susceptor for use in an induction heating device,

comprising: a first and a second plurality of conductive elongated elements, said first and second plurality of elongated elements having a length substantially greater than any dimension of the elongated elements cross section, said first plurality of elongated elements being connected together lengthwise to'form a plurality of endless structures, and said second plurality of elongated elements being connected lengthwise between said plurality of endless structures to form support means to space said endless structures from each other.

8. A susceptor in accordance with claim 7 wherein said plurality of endless structures and said second plurality of elongated elements are assembled to define a generally cylindrical structure.

9. A susceptor in accordance with claim 8 wherein said endless structures are spaced closer to one another at certain portions of said cylindrical structure than at other portions.

10. A susceptor in accordance with claim 8 wherein said endless structures adjacent the ends of said cylindrical structure are spaced closer together than the remaining intermediate endless structures.

11. A susceptor in accordance with claim 7 wherein said first plurality of elongated elements are connected to each other by tapered pins extending into tapered holes in the ends of said conductive elongated elements.

12. A susceptor in accordance with claim 7 wherein said second plurality of conductive elongated elements are provided with means at their ends for extending into openings in said first plurality of conductive elongated elements for supporting said plurality of endless structures from each otl1er. 

1. A susceptor for radiant heating, said susceptor comprising interconnected conductive bars, certain of said conductive bars being connected to each other to define a ring, a plurality of such rings, and certain other of said conductive bars interconnecting said rings and spacing said rings one from the other.
 2. A susceptor in accordance with claim 1 wherein said rings and spacing bars are assembled to define a generally cylindrical structure.
 3. A susceptor in accordance with claim 1 wherein said rings are spaced closer to one another at certain portions of said structure than at other portions.
 4. A susceptor in accordance with claim 1 wherein said rings are interconnected by said spacing bars to define a generally cylindrical structure, said rings adjacent the ends of said cylindrical structure being spaced closer together than the remaining intermediate rings.
 5. A susceptor in accordance with claim 1 wherein said certain of said conductive bars connected to each other to define a ring are connected to each other by tapered pins extending into tapered holes in the ends of said conductive bars.
 6. A susceptor in accordance with claim 1 wherein said spacing bars have means at their ends extending into openings in said certain of said conductive bars for supporting said certain of said conductive bars.
 7. A susceptor for use in an induction heating device, comprising: a first and a second plurality of conductive elongated elements, said first and second plurality of elongated elements having a length substantially greater than any dimension of the elongated elements cross section, said first plurality of elongated elements being connected together lengthwise to form a plurality of endless structures, and said second plurality of elongated elements being connected lengthwise between said plurality of endless structures to form support means to space said endless structures from each other.
 8. A susceptor in accordance with claim 7 wherein said plurality of endless structures and said second plurality of elongated elements are assembled to define a generally cylindrical structure.
 9. A susceptor in accordance with claim 8 wherein said endless structures are spaced closer to one another at certain portions of said cylindrical structure than at other portions.
 10. A susceptor in accordance with claim 8 wherein said endless structures adjacent the ends of said cylindrical structure are spaced closer together than the remaining intermediate endless structures.
 11. A susceptor in accordance with claim 7 wherein said first plurality of elongated elements are connected to each other by tapered pins extending into tapered holes in the ends of said conductive elongated elements.
 12. A susceptor in accordance with claim 7 wherein said second plurality of conductive elongated elements are provided with means at their ends for extending into openings in said first plurality of conductive elongated eleMents for supporting said plurality of endless structures from each other. 